The accuracy of integrated temperature sensors and ADCs can potentially be limited by semiconductor process parameter variations and/or environmental temperature variations. For example, integrated digital temperature sensors and ADCs that use a local voltage reference (such as a bandgap-based voltage reference) can be limited by the temperature variation of the voltage reference. Bipolar-based reference voltages may also be limited by the base-emitter voltage (VBE) variations among the devices in a batch. Bandgap-based voltage references can have both linear and nonlinear temperature variations due to device physics and manufacturing tolerances.
To achieve higher accuracy, temperature variations of the voltage reference can be compensated, corrected, or calibrated. In some cases, an analog correction scheme can attempt to provide an accurate voltage reference. Generally, analog correction techniques rely on circuit compensation and may use analog calibration at one or two known temperatures, incurring test time costs. Further, additional voltage reference circuitry may increase complexity and product costs. If a higher accuracy external voltage reference is used, this may go against system-on-chip integration standards, and may incur additional system costs and additional package pins.
Some digital correction techniques can also use calibration at one or two known temperatures as well as real time temperature information. Test time costs may be present, depending on the techniques used. For accurate post-processing, the temperature needs to be known at the time of the ADC measurement. This is generally done with an additional temperature sensor. Thus, the high accuracy temperature sensor can incur power and silicon area costs. Further, such sensors typically use an ADC with a voltage reference, and then the temperature variation of this voltage reference needs to be compensated or corrected as well.
In such a case, a classic “chicken and egg” problem is presented. To measure temperature accurately, the voltage reference temperature variation needs to be compensated, and that requires the accurate temperature to be known. For one solution, the voltage reference may be used without digital correction. However, this can degrade the accuracy of the temperature sensor and in turn can degrade the ADC reference correction accuracy. For example, in ADC performance, after reference digital correction using a temperature sensor without reference correction, a ±0.03% (3σ) gain error inaccuracy can be seen from −40 to +85° C.